It is known that lubricating oils suffer, during their use, changes from the lubricating process and as a result thereof the lubricating properties deteriorate. Some of the structural elements (for example, Perlite) passing into the oil during the movement of metal parts cause abrasive wear. Quartz, as the most frequently encountered part of solid air contaminants, also causes abrasive wear. The contaminants coming from construction material or passing from the outside into the system not only decrease the lifetime of the lubricated parts by abrasive wear but also act to catalyze the oxidation of the oil, since these impurities are excellent oxidation catalysts. In order to increase the lifetime of the lubricated parts and to avoid deterioration of the oil, it is necessary to filter the lubricating oils as completely as possible. If the presently employed fine oil filters operate free from defects, then the contamination of the oil amounts to several thousands of a gram per 100 ml. With such a degree of contamination, lubricating oils of the quality CD have to be changed every one thousand operating hours.
The time between two oil changes can be increased to 5000 to 10,000 hours of operation, as is to be expected, by reducing the degree of contamination of the oil by more effective filtration. For this purpose the degree of oil contamination has to be decreased to a few 10.sup.-4 g/100 ml. In the present description oil filtration of this quality is designated as super-fine oil filtration (SF-filtering). By changing the conventional fine filtering to super-fine filtering not only can the properties of the lubricating oil be more fully used and its lifetime increased, but also the wear can be decreased by reduction of the amount and size of the abrasive particles circulated with the motor oil and thereby the life time of the motor is favorably influenced.